The present invention relates in general to pressure sensing devices, and in particular to an ultra-sensitive pressure sensor with a thin boss structure and related methods of manufacture.
Pressure sensors can be found in a variety of applications including medical devices, automobile exhaust systems and gas tanks, industrial processes and consumer products such as pressure gauges. A common type of pressure sensor uses a silicon die that is micromachined to create a cavity in a central portion of the die that results in an area that is substantially thinner than the rest of the die. The thin area of the silicon die, commonly referred to as the diaphragm, is thin enough to deflect when subject to pressure. Piezo-electric resistors are then strategically located around the periphery of the diaphragm such that their resistance values change in response to the stress caused by the deflection of the diaphragm. The piezo-electric resistors are typically connected between a power supply and ground in a bridge network, and thus output a different voltage level in response to the variations in the resistance values.
The response of a pressure sensor is typically measured by its sensitivity and linearity. Sensitivity generally refers to the output voltage for unit pressure at unit supply voltage and is measured in mV/V/psi. In a conventional pressure sensor, a thinner diaphragm results in a device that can sense smaller amounts of pressure, but one that is much less linear. Therefore, overall performance (sensitivity and linearity) is not improved by simply making the diaphragm thinner. FIG. 1 shows the performance of two different pressure sensors. Sensor 1 is highly sensitive (steep curve) but has a linear range that is shorter than Sensor 2. Sensor 2, on the other hand, is less sensitive but has better linearity. A major challenge in designing pressure sensors is to simultaneously make the curve steeper and extend its linear range.
One approach that has improved the sensitivity of pressure sensors uses a bossed diaphragm. In this approach, when creating the cavity in the silicon substrate, a central portion of the region is masked such that the resulting diaphragm is in the form of a rectangular or square track (or an octagonal track). The masked portion that does not get removed from the center of the die is referred to as the boss. This center boss adds mass to the center of the diaphragm and essentially acts as a stress concentrator focusing the stress fields more toward the location of the piezo-resistors. The resulting structure suppresses nonlinearity of the output and results in a more sensitive device.
The bossed diaphragm structure has offered appreciable but still limited improvement in the performance of pressure sensors. There is always a need for pressure sensors with increased sensitivity to meet the demands of newer applications.